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[Source](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Symbol)
A symbol primitive represents a unique value that never collides with any other
value, including the value of other symbol primitives. Two string primitives
made up of identical characters evaluate as strictly equal:
```javascript
String() === String()
> true
String( "My string." ) === String( "My string." );
> true
```
However, no two symbols created using the `Symbol()` function can ever be
strictly equal:
```javascript
Symbol() === Symbol()
> false
```
This trait lets you use symbols as unique property keys in an object, preventing
collisions with keys any other code might add to that object.
```javascript
const mySymbol = Symbol( "Desc" );
const myObject = {};
myObject[mySymbol] = "propSymbol";
myObject
> Object { Symbol("Desc"): "propSymbol" }
```
There are three types of symbols:
- Symbols created with `Symbol()`
- Shared symbols that are set and retrieved from a global Symbol registry using `Symbol.for()`
- "Well-known symbols" defined as static properties on the Symbol object. These symbols contain internal methods that can't be accidentally overwritten.
`Symbol()` accepts a description (or "symbol name") as an optional argument.
These descriptions are human-readable labels for debugging purposes, and they
don't affect the uniqueness of the result. Any call to `Symbol` returns a
completely unique symbol primitive, even if multiple calls have identical
descriptions:
```javascript
Symbol( "My symbol." ) === Symbol( "My symbol." );
> false
```
As with other primitive data types, symbols inherit methods and properties from
their [prototype](https://web.dev/learn/javascript/appendix.md.txt#prototypal-inheritance). For example, you can access a description as an inherited property of the created symbol:
```javascript
let mySymbol = Symbol( "My symbol." );
mySymbol.description
> "My symbol."
```
But you can't create a symbol using the `new` keyword:
```javascript
let mySymbol = new Symbol();
> Uncaught TypeError: Symbol is not a constructor
```
Symbols aren't enumerable, meaning that symbolic properties aren't available
when using standard methods to iterate over them. The `getOwnPropertySymbols()`
method gives access to an object's symbol properties.
## Shared symbols
The `Symbol.for()` method tries to look up any existing symbols in a
runtime-wide global symbol registry with a given string as the key, and returns
the matching symbol if one is found. If it doesn't find one, it creates a symbol
with the specified key and adds it to the global registry:
```javascript
let sharedSymbol = Symbol.for( "My key." );
sharedSymbol === Symbol.for( "My key." )
> true
```
These keys share no functional overlap with the descriptions assigned to
author-created `Symbol` primitives. To access a symbol in the symbol registry,
you must first create it using `for()`:
```javascript
Symbol( "String" ) === Symbol( "String" );
> false
Symbol( "String" ) === Symbol.for( "String" );
> false
Symbol.for( "String" ) === Symbol.for( "String" );
> true
```
To retrieve the key for any symbol from the symbol registry, use
`Symbol.keyFor()`:
```javascript
let mySymbol = Symbol.for( "Key." );
Symbol.keyFor( mySymbol ) ;
> "Key."
```
## "Well-known" symbols
*Well-known symbols* are static properties of the `Symbol` object, each of which
is a symbol itself. Well-known symbols provide unique property keys for
accessing and modifying JavaScript's built-in methods, while preventing core
behavior from being unintentionally overwritten.
```javascript
Symbol;
> function Symbol()
asyncIterator: Symbol(Symbol.asyncIterator)
for: function for()
hasInstance: Symbol("Symbol.hasInstance")
isConcatSpreadable: Symbol("Symbol.isConcatSpreadable")
iterator: Symbol(Symbol.iterator)
keyFor: function keyFor()
length: 0
match: Symbol("Symbol.match")
matchAll: Symbol("Symbol.matchAll")
name: "Symbol"
prototype: Object { ... }
replace: Symbol("Symbol.replace")
search: Symbol("Symbol.search")
species: Symbol("Symbol.species")
split: Symbol("Symbol.split")
toPrimitive: Symbol("Symbol.toPrimitive")
toStringTag: Symbol("Symbol.toStringTag")
unscopables: Symbol("Symbol.unscopables")
: function ()
```
Because symbols are a feature specific to ES6, these symbolic values are
intended to be used as "extension points" for developers modifying JavaScript's
behavior without introducing backwards-compatibility issues.
Well-known symbol values are often stylized with a
[`@@` prefix or wrapped in `%`](https://github.com/tc39/ecma262/pull/1314) to
differentiate their keys from their mutable prototypes. For example, `@@match`
(or `%match%`) is a reference to the immutable `Symbol.match`, not
`String.prototype.match`.
## Check your understanding
Can you use `new` to create a symbol?
No Yes Which of the following describe \`well-known\` symbols?
Static properties of the \`Symbol\` object Symbols that you use frequently Unique property keys for accessing and modifying JavaScript's built-in methods